The Sedimentary and Geochemical Characteristics of the Upper Devonian Wutong Formation in Lower Yangtze Region
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摘要: 总结了下扬子区晚泥盆世五通组典型的沉积构造，如风暴砾滩、冲洗交错层理、滩脊、三脊波痕、植物化石及遗迹化石埋藏相特征等。全定量分析了泥岩类的主化学组成及微量元素，其主化学组成具有高SiO2含量。高K2O/Na2O比值，低MgO, Zr, Hf组合特征显示近源沉积特征，与下伏下古生界有显著差别，B, Ga, Ba, Sr, Rb等元素反映出滨海及海陆交互的沉积环境。综合分析沉积构造、沉积地球化学及区域地质特征，认为五通组形成于弧后前陆盆地，其沉积过程是一个海侵过程，这一大地构造背景一直延续到早石炭世末。Abstract: The integration between Lower Yangtze plate and South China plate took place at the end of Silurian.The integration brought about early to meso-Devonian stratigraphic hiatus.The lower Devonian Wutong Formation forms a connecting link between the preceding and the following sedimentation which recorded the tectonic evolution in this region.Our work on the sedimentary and geochemical characteristics of the Wutong Formation may plays an important role in studying the tectonic system transformation in this region. In this paper, the typical sedimentary characteristics of Wutong Formation, such as storm gravel bank, flushing cross-stratum, beach ridge, tricrest bed ripple, plant and trace fossil taphofacies characteristics, etc., has been recorded and summarized in detail. All these sedimentary characteristics show that the Wutong Formation is a set of littoral facies sedimentary formation and this sedimentary environment is far away from that of the underbedding lower Paleozoic. The main chemical composition and trace elements of mudstone have been quantitatively analyzed.Its main chemical compositions are characterized of high SiO2 and K2O/Na2O but low MgO. The great difference in chemical composition between Wutong Formation and the underlying lower Paleozoic formations suggest the tectonic system change at the beginning of early Devonian in Lower Yangtze region, and this conclusion is consistent with the result from the sedimentary characteristics and trace elements. In trace elements, Zr, Hf compositions distribute far away from that of upper Silurian in Zr-Hf graphs and shows to be proximal environment. B, Ga, Ba, Sr, Rb etc. and their relative ratio are characterized of littoral facies and paralic sedimentation. Summing up the sedimentary and geochemical characteristics and regional geology, the authors put forwards that the Wutong Formation is a set of littoral facies sedimentary formation that occurs in back-arc foreland basin, and this tectonic background continued to the end of early Carboniferous.
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Key words:
- Wutong Formation /
- Lower Yangtze region /
- back-arc foreland basin
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